This invention relates to well logging methods and apparatus and more particularly to nuclear well logging techniques to determine the presence of undesired water flow in cement voids or channels behind steel well casing in a cased well borehole.
Undesired fluid communication along the cased in portion of a well between producing zones has long been a problem in the petroleum industry. The communication of fresh or salt water from a nearby water sand into a petroleum production sand can contaminate the petroleum being produced by the well to an extent that production of petroleum from the well can become commercially unfeasible due to the "water cut". Similarly, in near surface water wells used for production of fresh water for city or town drinking supplies or the like, the contamination of the fresh water drinking supply by the migration of salt water from nearby sands can also contaminate the drinking water supply to the extent where it is unfit for human consumption without elaborate contaminant removal processing.
In both of these instances, it has been found through experience over the course of years that the contamination of fresh water drinking supplies or producing petroleum sands can occur many times due to the undesired communication of water from nearby sands down the annulus between the steel casing used to support the walls of the borehole and the borehole wall itself. Usually steel casing which is used for this purpose is cemented in place. If a good primary cement job is obtained on well completion, there is no problem with fluid communication between producing zones. However, in some areas of the world where very loosely consolidated, highly permeable sands are typical in production of petroleum, the sands may later collapse in the vicinity of the borehole even if a good primary cement job is obtained. This can allow the migration of water along the outside of the cement sheath from a nearby water sand into the producing zone. Also, the problem of undesired fluid communication occurs when the primary cement job itself deteriorates due to the flow of fluids in its vicinity. Similarly, an otherwise good primary cement job may contain longitudinal channels or void spaces along its length which permit undesired fluid communication between nearby water sands and the producing zone.
Another problem which can lead to undesired fluid communication along the borehole between producing oil zones and nearby water sands is that of the so called "microannulus" between the casing and the cement. This phenomenon occurs because when the cement is being forced from the bottom of the casing string up into the annulus between the casing and the formations, (or through casing perforations), the casing is usually submitted to a high hydrostatic pressure differential in order to force the cement into the annulus. The high pressure differential can cause casing expansion. When this pressure is subsequently relieved for producing from the well, the previously expanded casing may contract away from the cement sheath formed about it in the annulus between the casing and the formations. This contraction can leave a void space between the casing and the cement sheath which is sometimes referred to as a microannulus. In some instances, if enough casing expansion has taken place during the process of primary cementing (such as in a deep well where a high hydrostatic pressure is required) the casing may contract away from the cement sheath leaving a microannulus sufficiently wide for fluid to communicate from nearby water sands along the microannulus into the producing perforations and thereby produce an undesirable water cut.
There have been many attempts in the prior art to evaluate and locate the existance of cement channels. There have also been many attempts in the prior art to locate and confirm the existance of so called microannulus fluid communication problems. Perhaps primary among these attempts in the prior art has been that of the use of the acoustic cement bond log. In this type of logging operation, the amplitude of acoustic wave energy which is propogated along the casing from an acoustic transmitter to one or more acoustic receivers is examined. In principle, if the casing a firmly bonded to the cement and to the formations, the acoustic energy propogated along the casing should radiate outwardly from the casing into the cement and surrounding formations, thereby reducing the amplitude of the casing signal. However, if the casing is poorly bonded to the cement or if the cement is poorly bonded to the formations, a void space exists and the acoustic energy should remain in the casing and arrive at the acoustic energy receivers at a much higher amplitude than if a good cement bond existed between the casing, the cement and the formations.
Acoustic cement bond logging, however, cannot always reliably detect the existance of a microannulus which can in some instances permit undesirable fluid communication between water sands and nearby producing zones. If the microannulus is sufficiently small and fluid filled, the acoustic energy propagated along the casing may be coupled across it. Yet it has been found that even such a small microannulus can permit undesirble fluid communication between producing zones. Similarly, a poor quality cement job may go undetected by the use of the acoustic cement bond log if the cement sheath is permeated by a variety of channels or void spaces which are located unsymmetrically about its circumference. Such channels or void spaces can permit undesirable fluid flow while the main body of cement is bonded well to the casing and the formations thus propagating the acoustic energy satisfactorily from the casing outwardly through the cement and into the formations. Therefore, such means as acoustic cement bond well logging have been proven to be not entirely reliable for the detection of potential undesired fluid communication paths in a completed well.
Another approach to locating void spaces or channels in the cement sheath in the prior art has been to inject radioactive tracer substances such as Iodine 131 or the like through producing perforations into the producing formations and into any void spaces in the annulus surrounding the well casing. The theory in this type of operation is that if the tracer material can be forced backward along the flow path of the undesired fluid its radioactive properties may then be subsequently detected behind the casing by radiation detectors. This type of well logging operation has usually proven to be unsatisfactory however, particularly in loosely consolidated sand formations which is precisely where undesired fluid communication is most typically encountered.
In particularly permeable formations such as loosely consolidated sands, the producing formation itself can absorb most of the radioactive tracer material which is forced through the perforations. Very little, if any, of the tracer material can be forced back along the path of undesired fluid flow, particularly, if this involves forcing the flow of tracer against either formation fluid pressure or upward against the force of gravity. Therefore, such tracer logging techniques for detecting cement channels or voids behind the casing have usually proven ineffective in the prior art.